Visibility flags

[ChrissW-R1]

Adds feature to use WorldGuard flags to control visibility of region.
Regions, which are hidden by the config, will still be hidden. Also the defaults are set to work as the previous version of the plugin. If the configuration is not changed, everything remains consistent.

Added configuration options:

• visibility-by-flags.enable (Default: false) # Enable the feature to use WorldGuard flags to control visibility of regions
• visibility-by-flags.hide-by-default (Default: true) # Hide regions, without the 'dynmap-showonmap' WorldGuard flag

Added WorldGuard flag:
dynmap-showonmap:

• null / unset: Default from confguration will be used (visibility-by-flags.hide-by-default)
• false: Region will be hidden
• true: Region will be shown (if not hidden by previous system)

[ChrissW-R1]

Maybe solves #5 and #12